The present disclosure herein relates to a signal processing method of raw data obtained using a chirp sub-bottom profiler.
Unlike strata exposed to the surface, the sub-bottom has physical limitations in exploration. Therefore, sub-bottom exploration is mainly based on seismic surveys. In the seismic exploration, reflections or refractions that occur when the seismic signal artificially generated at the water reaches the sub-bottom are detected by a geophone, and the sensed signal is analyzed to analyze the sub-bottom. That is, by analyzing the detected signals, it is possible to probe the resources existing in the seabed such as oil, natural gas and gas hydrate, or to explore submarine cables, storage facilities, bridges and wrecks.
The seismic waves used in the sub-bottom may be roughly divided into low-frequency exploration and high-frequency exploration.
In the low-frequency exploration, there is a problem that the penetration of strata is higher than that of high frequency exploration, but the resolution of the data is low. Conversely, in the high-frequency exploration, there is a problem that the resolution of the data is higher than that of the low frequency probe, but the penetration of strata is low.
Therefore, strata exploration, located at a depth of several kilometers, uses a large-capacity air-gun that generates low frequencies, and strata exploration, located at relatively shallow water depths, uses small-volume air guns, boomers, multi-beams, side scan sonar, and chirp sub-bottom profiler (SBP).
Unlike other sound sources that use a single frequency, the dual chirp sub-bottom profiler uses a multi-frequency sound source that transmits and modulates frequencies from several kHz to several tens of kHz. That is, the multi-frequency sound source used in the chirp sub-bottom profiler has an advantage that it may grasp and analyze the high frequency band and the low frequency band relatively low at a time.
The chirp sub-bottom profiler emits a chirplet, that is, a signal that increases at low frequencies to high frequencies. The length of the chirplet is very short, ranging from several milliseconds to several tens of milliseconds, so that an anomaly of several tens cm or less may be distinguished.
However, since such chirp sub-bottom profiler also uses high frequency signals, as compared with the case of using a low-frequency signal, there are many problems that the impact of the marine environment is large, and when the signal is propagating to the medium, it becomes longer and a waveform distortion such as a change in amplitude and frequency occurs, and signal overlapping, noise, and the like occur greatly.
Also, because the chirp sub-bottom profiler uses a multi-frequency sound source that is modulated rather than a single-frequency sound source, it is difficult to process data. As such, due to the difficulty of processing chirp sub-bottom profiler data, conventionally, after performing a preprocessing process and a signal processing process to increase the S/N ratio, it is converted into an envelope type in which the energy intensity may be grasped and then, analyzed.
In such a manner, the method of converting the obtained data into the envelope form and analyzing it does not know the quantitative physical property information of the sub-bottom.
Therefore, in relation to the sub-bottom analysis method using the chirp sub-bottom profiler, there is a need for a data processing method capable of minimizing the effects of waveform distortion and simultaneously obtaining quantitative physical property information.